Pages

CV News

AAVSO Special Notice #404

July 10, 2015

V380 Oph in low state

The novalike/VY Scl variable V380 Oph is in its low state and is undergoing a deep fading, according to observations by Adolfo Darriba Martinez on 2015 July 4 and 5 reported via VSNET, CBA, and to the AAVSO International Database.

Dr. Joseph Patterson (Columbia University) writes: "[T]his seems to be a *fresh* fading - so in theory, it's now a bare white dwarf and an orbiting M dwarf. If the WD is hot (it should be), then the reflection effect off the M dwarf should produce a sinusoidal signal at Porb. So the amplitude of the signal furnishes a bolometer for the WD. It's an experiment we rarely get to do, because the accretion disk normally *shadows* the secondary from (most) WD light. So it's a great target for time-series photometry."

V380 Oph ranges from V~14.7 in its high state to as faint as V=19.3-19.4 in its low state. During this fading, AAVSO observers L. Cook (COO, Concord, CA) and G. Myers (MGW, Hillsborough, CA) have reported it as faint as CV=19.468 and CV=19.325, respectively.

The most recent observations reported to the AAVSO show it varying between CV = 18.694(100) and 18.993(109) on 2015 July 9.94917- 10.05814 UT (DAM, A. Darriba Martinez, Madrid, Spain).

CCD observers are requested to carry out time-series observations of V380 Oph. Due to the faintness of the target, unfiltered observations may be required, and are acceptable. Please clearly identify the comparison stars and magnitudes used for calibration.

Coordinates: RA 17 50 13.64 Dec. +06 05 29.4 (2000.0)

Finder charts with a sequence for V380 Oph may be created using the AAVSO Variable Star Plotter (VSP, http://www.aavso.org/vsp). A photometry table of the sequence is also available via VSP.

Please report your observations of V380 Oph to the AAVSO International Database via WebObs using the name "V380 OPH".

V404 Cyg has been observed by the professional community across thespectrum from X-ray to radio wavelengths, and by the amateurcommunity in all bands available to it. Large- and small-scalevariations have been seen occurring at timescales ranging fromseconds to hours, with changes of 2-3 magnitudes seen over thecourse of a few hours. Most recently Swift has detected a dust haloaround V404 Cyg (Beardmore et al., ATel #7736,http://www.astronomerstelegram.org/?read=7736).

C. Knigge (University of Southampton) et al. announce (ATel #7735,http://www.astronomerstelegram.org/?read=7735) that Far-ultraviolet(FUV) and near-ultraviolet (NUV) spectroscopic observations of V404Cyg with the Hubble Space Telescope have been scheduled for 2015July 10, 11, and 12 according to the schedule below:

The HST observations were scheduled for the satellite's earliestavailability, and the timing is best for ground-based locationsfrom the western US and eastern Asia.

Regardless of location, all observers are requested to continue toobserve V404 Cyg. Time-series observations are requested during asmuch of the HST observation intervals as possible, in as many bandsas possible. A high cadence is recommended in order to detect thevery short-period variations that have been seen.

Koji Mukai

Koji Mukai was born in Osaka, Japan, and graduated from University of Tokyowith a Rigakushi (B.Sc.) degree.

For his graduate study, he chose University of Oxford. There, he worked withPhil Charles, Robin Corbet and Alan Smale, among others, on interactingbinaries, completing his Ph.D. thesis on AM Her type systems in less than 3years and 1 month. He then spent 3 years at Mullard Space ScienceLaboratory, working with Keith Mason, Alan Smale, Simon Rosen and CoelHellier.

During his 6 years in England, he was mostly an optical astronomer, and had6 observing trips to La Palma, 3 to South Africa, 2 to Australia, and 1 toHawaii, using world-class telescopes with state-of-the-art instrumentslocated at the best sites in the world.

After moving to the United States, Koji spent 2 years at UC Berkeley'sCenter for EUV Astrophysics, working on preparations for the EUVE all-skysurvey. In January 1992 Koji joined NASA Goddard Space Flight Center as anUSRA research scientist. Initially, his work was on the Japanese-US ASCAmission. More recently he has worked on the ill-fated ASTRO-E mission, andnow the Astro-E2 mission, renamed Suzaku after launch.

In recent years, Koji has focused his research efforts into the X-ray observations of magnetic cataclysmic variables and other accreting binaries.It is these magnetic CVs, specifically intermediate polars (IPs) that willbe the topic of out interview.

CVnet: Hello, Koji. Thank you for agreeing to participate in this CVnetinterview series.

IPs are the CV class with the most ferocious debates about membership ornot. On your web site you refer to yourself as the 'Curmudgeon', referringto your strict requisites for inclusion as a member of the IP class, so let's begin with the definition of an intermediate polar.Intermediate polars are systems between non-magnetic CVs and the AM Hersystems. The accretion process in IPs is through a disc with a disruptedinner radius, or an accretion stream (as in the polars), or both. By virtueof a lower magnetic field (as compared to polars) the accreted area on thewhite dwarf is larger, and typically extends over a hemisphere. The whitedwarf spin is not synchronous with the orbital period, as in the AM Hersystems. The spin period of the white dwarf is shorter than the orbitalperiod.What other characteristics would you include to define a "genuine IP"?

Mukai: Nothing at all. The definition is fine, I think pretty mucheverybody agrees. Any differences of opinion are in how to applythis definition to real-life data. What I'm trying to do, as theCurmudgeon, is to curb the enthusiasm of some observers. I mean,if you see a peak in the periodogram of a three-hour stretchof photometry of a CV, you shouldn't jump to the conclusion thatyou've discovered an IP. After all, all CVs vary on a variety oftimescales, and that will produce a peak in the periodogram somewhere.You need more than just half a night's photometry to be sure you'relooking at a persistent, periodic, characteristic of the system.

CVnet: The intermediate polars with the shortest spin periods and weakestmagnetic fields are called DQ Herculis stars, although there has been somediscussion concerning whether they really deserve to be called a separateclass of object. Are all DQ Her systems IPs, are all IPs DQ Hers, or isthere a distinction?

Mukai: I personally don't think there are two distinct classes - thereare quantitative differences, yes, but not qualitative one. So, to me,they are all IPs. You can call them all DQ Her systems, like Joe Pattersonprefers to do.

CVnet: How does the relative stability of the spin period provide evidencefor white dwarf, as opposed to neutron star, nature of the compact object.

Mukai: Speaking of Joe, I think he was the first person to point this out,way back in 1981 in his Nature paper. Basically, if you compare the radiusof a white dwarf versus that of a neutron star, a white dwarf is about athousand times bigger. And that makes them more stable - it's much harderto change the spin of a white dwarf than the spin of a neutron star. So,observationally, if you see the spin of an X-ray pulsar changing quickly,then you suspect it's a neutron star. If the spin doesn't change much,either you were unlucky or the system has a white dwarf.

CVnet: Will you explain what a spin up or down is, and how it relates to thestudy of these systems?

Mukai: If you measure the spin period of an IP accurately one year,and come back the following observing season, you might find thatthe spin period is now slightly shorter - that's spin up - or slightlylonger - that's spin down. Since white dwarf spin is stable, it takesyears of collecting a lot of photometry to really see if an IP is spinningup or spinning down. In some cases, one system might do both!

IPs probably last hundreds of millions of years as IPs, graduallychanging its orbital period and such. If IPs never deviate from such longterm average, it should be exceedingly difficult to detect any spin upsor downs. The fact that we do see them mean that IPs do deviate fromtheir long-term average on timescales shorter than, say, millions ofyears. In simple terms, if the accretion rate goes up, the torqueexerted by the accreting gas wins and the white dwarf spins up.If the accretion rate goes down, the braking effects of the rotatingmagnetic field wins and the white dwarf spins down. But the physicsis rather complicated - any time you have an interaction of plasma andmagnetic field, it's hard to figure out exactly what will happen (justask the people trying to build nuclear fusion reactors!). So, gettingnumbers out of spin ups and spin downs is not easy - or rather, you canextract numbers but you never know how much you can trust them.

CVnet: Some IPs, [GK Per, DO(YY) Dra, HT Cam, and EX Hya] , exhibitoutbursts. These are probably the systems most familiar to CVnet observersand participants. What is the current thinking on the cause of theseoutbursts, and why are these systems different than the IPs that don'texhibit outbursts?

Mukai: You would think that the mechanism is the same for these IPsas in ordinary (non-magnetic) dwarf novae. By that logic, the IPoutbursts must also be due to instability in the accretion disk.

CVnet: Some IPs have shown occasional low states in archival platephotometry, but probably not as frequently as in AM Her type systems. Whatare the possible causes for this behavior?

Mukai: I'm mostly going to punt on this question, because nobody reallyknows why some CVs go into low states. Yes, some clever people have ideas,but there are far more questions than answers on this topic, in my opinion.There are some ideas why AM Her type systems go into low state more often- perhaps because they don't have a reservoir of mass in the form of anaccretion disk, perhaps because the magnetic field would force the plasmato climb up the gravitational potential (that's hard to do!) under certainsituations.

But mostly, I'd like to appeal to the readers of CVnet to watch out forIPs going into low states. Because, as far as I know, the only knownexamples of IPs going into low states are from archival plates, discoveredmany years after the fact, and nobody has caught one in action. Low states,if you can catch one, are a great opportunity to learn about the underlyingstars uncontaminated by accretion. That would be great! In contrast, AM Hertype systems go into low states so often that many of us now consider lowstates to be an annoyance as far as AM Her systems are concerned.

CVnet: Let's talk about the evolutionary track of these systems a bit. Thecommonly proposed evolutionary history of non-magnetic and magnetic CVs hasbeen assumed to be similar. Recent results, infrared spectroscopy of twodozen CVs in Harrison et al (2004, 2005) and UV spectroscopy by Gaensicke etal 2003, find evidence for peculiar abundance ratios in the secondaries ofnon-magnetic CVs; specifically deficits of carbon and enhancements ofnitrogen. If magnetic CVs follow the same evolutionary path, one wouldexpect to find similar abundances in the secondary stars of magneticsystems. In Harrison et al, Astrophys.J. 632 (2005), severalpolars were examined and the spectra of these secondaries are consistentwith normal late type dwarfs, suggesting that the evolution of secondariesin magnetic systems is different than that of non-magnetic systems. Have there been similar studies of the secondaries in IPs, and if so, arethe secondaries of these systems normal late type dwarfs or do they exhibitthe same peculiar abundance ratios as non-magnetic systems?

Mukai: By and large, the secondary of IPs have never been convincinglydetected - with the exception of GK Per and YY Dra, I think. Accretionrates are so high in IPs, it's hard to see the secondary (and believe me,I've tried). That's another reason to want to see them go into low states.

CVnet: One of the goals of CVnet is to bring professional and amateurastronomers together to the mutual benefit of both. What can interestedamateur observers with modest telescopes do to help in the understanding ofthese systems?

Mukai: CVnet observers can watch out for outbursts and low states ofIPs - at the moment, I don't have anything specifically set up to observeoutbursts, but it's always useful to know. As I said, if an IP is foundin a low state, that can be a gold mine of information. The other mainthing is to follow the spin ups and spin downs - the best way may be tojoin Joe Patterson's CBA network, which always have campaigns set up tomake sure there are enough data on IPs to be able to track their spinperiods.

CVnet: Thank you very much, Koji. I hope you enjoyed this and will agree totalk with us again in the future.